Soaking pit and method of operating the same



Aug. 20, 1968 A. AUGUSTINE SOAKING PIT AND METHOD OF OPERATING THE SAME3 Sheets-Sheet 1 Filed Oct. 28, 1966 INVENIOR ALFRED AUGUSTI N E.

ATTORNEYS.

Aug. 20, 1968 A. AUGUSTINE 3,397,874 v SOAKING PIT AND METHOD OFOPERATING THE SAME 3 Sheets-Sheet 2 Filed Oct. 28, 1966 RR. 2 m mm. 2 4V6 7 N U A F m N J E 47 3 .4 A 5 2 WM 1 r ATTORNEYS.

Aug. 20, 1968 A. AUGUSTINE SOAKING PIT AND METHOD OF OPERATING THE SAMEINVENTOR ALFRED AUGUSTlNE.

BY M,MIMJ

ATTO RN EYS.

United States Patent 3,397,874 SOAKING PIT AND METHOD OF OPERATING THESAME Alfred Augustine, Pittsburgh, Pa., assignor to Loftus EngineeringCorporation, Pittsburgh, Pa., a corporation of Maryland Filed Oct. 28,1966, Ser. No. 590,265

Claims. (Cl. 263-40) ABSTRACT OF THE DISCLOSURE A soaking pit having acover is provided that is movable horizontally from a position over thepit to a position clear of the pit without the necessity of also raisingand lowering the cover as is the case where conventional sand seals areemployed. When the cover is over the pit in operating position there isan open gap between the cover and top wall of the furnace around theentire periphery of the furnace. An air duct system extending around thegap projects a high velocity curtain of air across the gap in suchmanner as to form a pressure barrier against the escape through the gapof furnace gases but the air flows outwardly around the outer peripheryof the gap, and not into the furnace. The invention not only eliminatesthe usual sand seals and the need for raising or lowering the cover, butprovides a new method of operating a soaking pit since higher pressurescan be maintained under the cover than with sand seals, making itpossible to maintain a positive pressure inside the pit from the coverto the level of the pit bottom.

This invention relates to industrial furnaces of the type known assoaking pits, in which metal ingots are heated to rolling temperature,and to the method of operating such furnaces.

Soaking pits have a bottom, and enclosing walls, and they are providedwith a cover which is movable from a position over the furnace to aposition clear of the top to enable ingots to be placed in the furnaceor removed therefrom. As now constructed the cover is suspended from acarriage by which it is traversed into and out of position over thefurnace, and the suspension is of a type which enables the cover to belowered after it is brought into position over the furnace and liftedclear of the furnace when it is to be moved from over the furnace. Thetop of the furnace is provided about its periphery with a trough toretain sand and the cover has sealing plates depending from itsperiphery that penetrate the sand in the trough to form a seal betweenthe cover and the top of the furnace when the cover is in closedposition. The raising of the cover before moving the carriage is madenecessary so that the sealing plates will first be lifted clear of thesand seal and furnace walls. Whether the furnace is completely opened oronly partially opened, this lifting of the cover is necessary.

The burner arrangements for these furnaces vary with the shape of thepit. These furnaces are usually rectangular and in recent practice haveone-way firing with a fuel burner or burners firing horizontally throughone end wall above the ingot charge. Burned gas outlet ports are locatedbelow the burner in the same end wall. In some cases they have two-wayfiring where there are burners and burned gas outlet ports below theburners in both end walls. In any case, because the ingots are chargedinto and removed vertically from the top, the outlet for burned gases isnot through the cover.

Pits vary in depth, some being considerably deeper than others. Themaximum pressure in the furnace is always at the top, because of thehottest gases rising, and the amount of pressure above atmosphericpressure 3,397,874 Patented Aug. 20, 1968 ice is limited by the sandseal to a fraction of an inch of water, something of the order of .06inch to about .09 inch at the seal level. Since the highest pressure isat the top, immediately under the cover, the pressure decreasesdownwardly and there comes a level between the top and bottom where thepressure is zero, that is, equal to atmospheric pressure, and below thislevel the pressure is negative relative to the atmosphere outside thefurnace. Soaking pit gases at 2400 F. develops a buoyancy of .012 inchwater per foot of height. Then the zero level in a pit fifteen feet indepth, for example, is about 7.5 feet below the cover when the pressureat the level of the top is of the order of .09 inch of water. At apressure of .18 inch of water top level pressure, the zero pressurelevel is fifteen feet below the cover and the entire furnace would be atpositive pressure. The same conditions may be figured of course forother depths. It will therefore be seen that by increasing the pressureat the top of the furnace, a point may be reached where the entireinterior of the furnace is at atmospheric pressure or above. However, ifthe usual soaking pit furnace which usually range between 10 and 15 feetin depth were so operated, the sand seal would be ineffective, therewould be a hazardous condition of sting-outs, and the sealing plateswould require replacement much more frequently than is now the case.

The importance of maintaining the point of zero pressure at the bottom,or having the entire interior of the furnace at positive pressurerelatively to the surrounding atmosphere is that when the pressureinside the furnace at any level is negative, air seeking to relieve thepartial vacuum is drawn into the furnace through its refractory walls orother source of leakage, and this air, becoming heated, oxidizes theingots, increasing scale formation.

A further phenomenon of these furnaces, particularly when operating'withhigh velocity burners, is the pulsating character of the flame.Recording devices that register pit furnace pressure fluctuate rapidly,tracing a continuous sinuous path a half inch or more in width. Thisreflects a condition where the automatic furnace pressure control dampermust continuously hunt to maintain even pressure conditions in thefurnace.

It may be noted that the maintenance of the sand seals, bv reason ofwarpage and deterioration of the sealing plates on the cover,constitutes a cost of several thousand dollars a year where there is theusual battery of several pits, and this expense does not include downtime for the making of repairs.

The present invention has for its principal object to provide a seal forsoaking pit covers and a method of operation which will enable higherpressures to be maintained in the pit without the escape of flames fromunder the cover or the inflow of air into the furnace, and which willsubstantial-1y reduce the amplitude of furnace pres sure pulsations,providing improved stability of the furnace pressure control. A furtherobject is to eliminate sand seals and sealing plates, and in so doingeliminate the need for raising and lowering the cover, thereby alsosubstantially reducing the cost of the carriage and cover arrangementand reducing the time cycle for opening or closing the furnace.

A further object is to provide a seal which may be applied to existingpits as well as to new construction and will reduce the cost of sealmaintenance. Moreover it may be adjusted to meet the irregularities inthe covers or wall conditions of diflerent furnaces due to camber or sagin the covers or erosion of refractory at the top of the furnace.

These and other objects and advantages are secured by my inventionwherein the cover is maintained at all times at a substantially fixedlevel above the top surface of the furnace wall, this surface beingcommonly referred to as the curb or coping. The usual sand trough aroundthe curb and seal plates on the cover are omitted. Provision is made formaintaining a curtain of high velocity air across the gap between thecover and the furnace curb. The kinetic energy of the air beingconverted to pressure creates a barrier in the gap between the aircurtain and inner periphery of the gap which is balanced against theinternal pressure under the cover so that the air prevents the escape offurnace gases, but the air itself flows outwardly away from the furnaceat a moderate temperature, even though the interior of the furnace isheated to iucandescence. The air curtain is generated by a duct systemaround the periphery of the furnace or cover, preferably the cover, towhich air under pressure is supplied. This duct system is comprisedprincipally of longitudinally-slit pipe sections with means foradjusting the width of the air discharge slit in each section, and eachsection is desirably independently rotatable about its axis to adjustthe angle of discharge of air therefrom. This is made possible by havingthe ends of each pipe section received in connectors or plenums throughwhich air under pressure is introduced into the duct system at intervalstherearound.

The invention may be more fully understood by reference to theaccompanying drawings illustrating a present preferred embodiment of theinvention, and in which:

FIG. 1 is a side elevation of a soaking pit, cover and cover carriagewherein the soaking pit is shown in longitudinal section and the coveris shown in side elevation, portions of the pit and cover being brokenaway to better illustrate the invention;

FIG. 2 is a top plan view of the cover and air supply system detachedfrom the carriage;

FIG. 3 is an end view of FIG. 1 with only a portion of the carriagebeing illustrated, the top only of the furnace walls being shown insection;

FIG. 4 is a schematic view showing one arrangement of the air ductsystem removed from the cover;

FIG. 5 is an enlarged detail of the connection between one of the airsupply ducts and the duct system around the cover, the view being partlyin section and partly in elevation;

FIG. 6 is an enlarged section through one of the slit pipes of the ductsystem with the outline of the top of the furnace wall and the adjacentpart of the cover, the section being in the plane of line VI-VI of FIG.2; and

FIG. 7 is a detail view showing an end view of one of the slit pipes.

In FIGS. 1 and 3 of the drawings there is illustrated more or lessschematically a soaking pit furnace of rectangular shape having a bottom2, end walls 3, and side walls 4 which are formed prinicpally ofrefractory, and which enclose the furnace pit 5. Generally there is alayer of loose refractory material 6 over the bottom of the furnace onwhich the ingots or similar objects to be heated are set. The end wallsand side walls terminate in a substantially flat level surface, hereinsometimes referred to as the coping or curb.

In the construction shown, the cover designated generally as 7 issuspended by links 8 from an overhead carriage 9 that has wheels 10 formovement along a track 11 at each side of the furnace. In some cases itmay be desirable to eliminate the carriage and put the wheels and theirdriving gear directly on the cover, a modification made possible by thisinvention.

The construction of the carriage and its driving mechanism form no partof the present invention, but it may be explained that it is common inthe art to provide remotely controlled motors on the carriage fortraversing the cover endwise from a position over the furnace to aposition at one end thereof with other motors on the carriage forlifting and lowering the cover for clearing or entering the sand seals.It will later be 7 seen that this invention eliminates any need for thisraising and lowering of the cover. Soaking pits of this type are chargedby traversing the cover to an open position and introducing the ingotsthrough the open top of the furnace, and in like manner the ingots,after being heated, are removed through the top after the cover has beenremoved.

The cover 7 itself comprises a rectangular steel framework that includeslongitudinally-extending side beams 12 and cross beams 13 at each end.The steel framework, which may take various forms, supports a continuousrefractory cover-forming body only fragmentarily indicated at 15 inFIGS. 2 and 6.

As shown in FIGS. 1 and 3, the cover, when it is in closed position, isover the open pit with the sides and ends of the covers projecting partway over the coping or rim of the furnace, and spaced above it anaverage distance under ideal conditions of about 1 /2 inches. Inpractice this distance may vary since the length of the pit is usuallyin excess of 20 feet, and of somewhat lesser width. Under theseconditions the metal framework will be subject to expansion andcontraction, and it is customary in the art to put a bow or camber inthe beams 12 and to a less extent in the cross beams 13 so that when thecover is cold it bows upward to a slight extent, and as it elongatesunder increased temperature it may therefore straighten without sagging.By reason of this and by reason of erosion or spalling that may occuralong the rim of the furnace this distance of 1 /2 inches does notremain ideally constant and may vary from point to point along thelength and width of the furnace.

In the construction shown there is a duct system supported on the underside of the structural members 12 and 13 and recessed so that the ductis shielded by the cover from exposure to heat radiated from theinterior of the furnace through the gap between the cover and furnacerim. This system is comprised principally of pipe sections 16 having alongitudinal slit 17 therealong. This is best shown in FIG. 6. Atintervals along the length of each pipe section there arediametrically-extending bolts 18 with nuts at one end thereof. Bytightening or loosening these nuts, the width of the slit 17 may bevaried, the tightening of the nuts tending to close the slit, whereas ifthe nuts are loosened the pipe tends to spring back or may be forcedback to its normal diameter. Wedges may be used in the slits 17 to gaugethe degree of closing or opening. One end of each pipe section is closedas indicated at 19 in FIG. 4, while the other end is received in a glandportion 20 of a connector 21 which also constitutes a plenum chamber. Inthe particular arrangement shown in the drawings, there are two of thesesplit pipe sections at each end of the cover extending horizontally fromthe connector 21 at the middle of the cover. At each side of the coverthere are two of these connectors 21, each with two pipe sections 16,one of which extends in a direction opposite the other. At the mid pointof the furnace the confronting closed ends of the tubes are practicallyin end-toend contact. The connectors 21 may each have a series of smallopenings 21a in closely-spaced relation arranged in a row along thebottom of the connector.

By adjusting the glands 20, the individual sections of pipe may berotated in the connector so as to adjust the position of the slot orslit 17 to direct air at the desired angle toward the furnace rim orcoping. Each connector 21 has a down pipe 22 secured thereto, each downpipe forming the terminal portion of a feeder line 23 leading from aplenum chamber 24 that is positioned on the cover. There is a butterflyvalve or other flow control valve 25 in each of these feeder pipes,these valves being here shown as being at the tops of the respectivedown pipes 22.

It will thus be seen that each connector 21 supplies air to two splitduct sections 16 and there is a separate feeder pipe leading from theplenum chamber 24 to each connector.

In the particular arrangement shown where the cover is suspended fromthe carriage, air is supplied to the plenum 24 by means of amotor-driven blower 26 supported on the carriage with its outlet 27connected at the inlet 28 of the plenum 24 through a flexible duct 29 ofthe bellows type. Under ideal conditions where the curb or coping of thefurnace is in good smooth condition, and where the cover maintains asubstantially uniform spacing between the cover and the curb, it isdesirable that the pipes 16 be set in the approximate position shown inFIG. 6 where the discharge of air through the slot 17 against the curbis directed downwardly and inwardly to substantially bisect a verticalline drawn from the outer edge of the cover to the curb. In theparticular instance shown, this angle is approximately 30 off verticalso that the resulting curtain of air is not vertical, but projectsinwardly under the cover. When the air encounters the curb, it appearsthat a substantial part at least is deflected upwardly and inwardly, itskinetic energy being transformed to pressure, thereby forming a barrieror dam in the gap between the inner and outer peripheries of the gapbetween the two confronting surfaces. The air then escapes to the outerperiphery of the gap. This pattern of air flow is evidenced by a visibledemarcation between refractory on the furnace rim being at a visiblyglowing temperature along a line inwardly in the gap from the line wherethe air actually impinges on the rim at some point between the aircurtain and the inner periphery of the gap. At any rate there is nosting-out of flame or hot gases where this condition prevails, and apiece of paper held in the stream of outflowing air at the edge of thecurb is not charred by the air, although it may, after a period ofseveral minutes, become charred primarily from the radiation of heatfrom the interior of the furnace. Likewise ones hand may be broughtclose to this position without serious discomfort.

As a specific example, the arrangement above described was applied to apit having an overall depth of about 14 ft. on the bottom of which wasplaced a layer of granular magnesite about 15 in. in depth. The pit wasof rectangular shape having an interior length of 27 ft. and a width of9 ft. The cover from the center line of the pipe 16 at one end to thecenter line of the pipe 16 at the other end was approximately 29 ft. 2in. and the width of the cover from thecenter line of pipe 16 at oneside to the center line of the corresponding pipe at the other side was11 ft. 2 in. The perimeter of the cover overhangs the perimeter of therim, but not to the full outer periphery of the furnace walls which wereof such thickness that they extended outwardly beyond the center linesof the pipes or ducts 16 at both the sides and ends, in the relationshipshown in FIG. 6. In this installation the overall length of the combinedpipes 16 and the connectors representing the perimeter of the cover atthe center line of the pipes 16 was about 80 ft. 8

The pipes constituting the duct system around the periphery of the coverwere 3 in. inside diameter, and the maximum opening of the slot was /8in. The inside diameter of the feeder and down pipes was 4 in. Theopenings 21a in the connectors were about A; in. spaced about onediameter apart. The pipes 16 at the corners practically abut so thatthere is no break in the air curtain.

The blower was driven by a 7 /2 horsepower motor operating at a speed of3600 rpm. and was rated to produce 2300 c.f.m. at oz. pressure in theplenum 24, and in this example was so operated. The valves 25 betweenthe feeders and the down legs 22 were adjusted to provide a pressure ineach of 3.5 oz. and an internal static pressure was maintained in thepipes 16 of 2.3 oz. The furnace was operated with a charge of ingots inthe usual manner. In the particular instance referred to, the curb hadbeen spalled in some places and the cover, upon heating, developeduneven spacing between the cover and the curb, but in general thedistance of the gap separating the cover and furnace curb was about 1 /2in. In no case was the space more than 3 in. With an initial furnacepressure under the cover at the seal of about .09 in. of water, therewas initially some sting-out. Where there was a bad condition ofspalling, the pipes 16 were rotated slightly to increase the angle ofdischarge of air further in toward the furnace pit, or the nuts on thebolts 18 of certain sections were adjusted to restrict the slit from in.to A in. By making these adjustments or combinations of them, allsting-out was eliminated. The use of slits 17 is preferable to the useof closely-spaced small holes, similar to holes 21a in the connectors,which would be used in pipes 16, but small holes do not provide the easeof adjustment.

After a period of operation the pressure inside the furnace wasincreased so that the pressure at the seal inside the furnace was .15in. of water and there was still no evidence of sting-out. This pressureat the seal of .15 in. of Water was more than adequate to produce apositive pressure in the pit to the full depth of the loose material atthe bottom of the pit.

It was observed that when the furnace was in operation there was astraight line of demarcation apparent on the coping or curb of thefurnace a short distance inwardly from where the air impinged, therefractory being red inside the line and black outside the line,indicating that the air current was uniformly effective, and that thehot gases were not lapping out quite to the point of impingement of thehigh velocity air stream against the refractory.

The shape and dimensions of the furnace above described is typical ofmost soaking pit furnaces. Generally they are rectangular and generallythey range between 10 and 15 ft. in depth so that the conditions ofoperation in the above example would be applicable to most soaking pits.For soaking pits of greater depth than 15 ft., one skilled in the artmay be. required to make some changes. A deep pit, for example, may havegreater dimensions in which greater fan capacity would be required tomaintain the desired velocity of the air curtain, but on the basis ofthe information herein given, one skilled in the art would have nodifficultay in adapting such a furnace for the practice of theinvention.

While I am aware that so-called air curtains have been used in variousenvironments such as doorways through which passage of persons orobjects occurs with considerable frequency, such installations commonlyoperate in the plane of the opening and are not angled inwardly as inthe present invention, and ordinarily atmospheric pressure prevails bothinside and outside the doorway. So far as I am aware, it has heretoforenever been considered that such a curtain would be effective to enablethe pressure inside a soaking pit to be increased above the normalpressure and prevent sting-outs at the higher pressures. In the examplesa'bove referred to, it was also observed that whereas the burnerpulsated with rapid explosions such as to cause the pressure-recordingindicator to oscillate rapidly over a band wider than /2 in. with aconventional sand seal, the same recorder, when the air curtain wassubstituted, reduced the magnitude of the oscillations to perhaps aroundA in. and the furnace pressure control was noted to be much smoother andquieter, and the stack damper, instead of continuously hunting, stayedmuch more uniformly at one position. Apparently the barrier generated bythe air curtain provided a resilience or elasticity which the sealingplates and sand seal could not provide.

It will be seen that with this invention the kinetic energy of the airis utilized to oppose the escape of gases from the interior of thefurnace, and the nice balance of pressures that would be required with astatic air seal is not necessary. The cost of continuously operating theblower is less than the cost of maintaining the seal plates and sandseal of a conventional cover in operating condition, and is much moresuccessful in eliminating stingouts. The scale on the ingots beingheated appeared to be of less thickness than with the conventional sandseal and to test this out a companion soaking pit of identicalconstruction but having the conventional sand seal was operated alongside of the one having the present invention experimentally appliedthereto. This is perhaps due to the more effective prevention ofescaping combustion gases with the seal of the present invention and thereduced influx of air at levels below the level of zero or atmosphericpressure explained above.

The invention simplifies the construction of the carriage and coverbecause of the elimination of expensive hoisting mechanism for raisingand lowering the cover which in itself weights several tons, since thecover can be traversed without any vertical movement being necessary.Time lost in repeated raising and lowering of the cover is eliminated.Closer integration between the carriage and cover becomes possible to apoint where they may be combined into a unitary structure in new furnaceconstruction.

While I have shown and described a preferred embodiment of my inventionand a method of operating the same, it will be understood that variouschanges and modifications may be made. In some cases it may even bedesirable to project the air stream from the slitted duct sectionsupwardly from ducts located in the top of the furnace wall with airimpinging against the cover. Also, the soaking pit herein described isof a type commonly used for heating steel, but the invention isapplicable to soaking pits as used in other industries, as for example,aluminum, when lower temperature and different firing methods areemployed.

These and other changes and modifications may be made within thecontemplation of my invention and under the scope of the followingclaims.

I claim:

1. A soaking pit comprising a pit furnace having a bottom andsurrounding walls forming an enclosure in which objects are placed to beheated,

a cover structure coextensive with the area of the enclosure andprojecting over the rim of the furnace walls when it is in closedposition for confining combustion gases therebeneath,

means for supporting the cover for horizontal travel from a positionover the furnace to a position clear of the top of the furnace,

said cover-supporting means being arranged to hold the cover in closedspaced relation to the rim when the cover is in furnace-closing positionwhereby there is a continuous open gap under the cover above the rim ofthe furnace, and

means arranged for projecting a continuous curtain of air across saidgap around the furnace between the rim and cover in such manner as toform a pressure barrier in the gap between the air curtain and the innerperiphery of the gap to prevent the escape of hot gases between thecover and the rim of the furnace but with the air escaping only at theouter periphery of the gap.

2. A soaking pit as defined in claim 1 in which said last-named means isarranged so that the air flow forming the curtain is angled inwardlytoward the furnace pit from said means for projecting air across thegap, whereby the kinetic energy of the air is transformed into pressureto form a barrier inwardly of the angle of incidence of the air againstthe surface toward which it is directed.

3. A soaking pit as defined in claim 1 in which said last-named meanscomprises a peripherally-extending duct system comprising sections oflongitudinally-slitted pipes,

connectors for delivering air to said pipes, feeder pipes leading to theconnectors,

means for supplying air under pressure to the feeder pipes, and

means for regulating the air flow through each feeder pipe.

4. A soaking pit as defined in claim 3 wherein means is provided forvarying the width of the slits in said slitted pipes.

5. A soaking pit as defined in claim 3 in which said 8 slitted pipes arerotatably adjustable about their respective longitudinal axes.

6. A soaking pit as defined in claim 3 in which said duct system,connectors, feeder pipes and means for supplying air to the feeder pipesis mounted on said cover.

7. A soaking pit as defined in claim 3 in which the connectors areperforated to project an air curtain therefrom throughout their lengths.

8. In the combination with a soaking pit having an open top furnace witha bottom and vertical walls enclosing a pit, and a cover that issupported for movement from a position over the furnace to a positionclear of the open top of the furnace with the cover at a level spacedfrom the top of the furnace, there being spaced confronting surfaces onthe cover and top of the furnace walls when the cover is over thefurnace, with a gap around the furnace ranging between about 1 /2 inchesand 3 inches between the confronting surfaces, the invention comprising:

a duct system arranged to project a curtain of air across the gapbetween the top of the furnace structure and the cover inwardly at anangle to a direction normal to the confronting surfaces entirely intothe gap about the perimeter of the walls and cover when the cover ispositioned over the furnace,

said means comprising a series of separate pipe sections, each sectionhaving a longitudinal slit extending therealong,

means for supplying air under pressure to said pipe sections, and

means for regulating the pressure in some of said pipe sectionsrelatively to the others.

9. The method of operating a soaking pit having a bottom, side walls andan open top and a cover removably positioned over the top with theperiphery of the cover overhanging the top of the furnace walls butspaced between about 1 /2 inches and about 3 inches from the top of theside walls of the furnace leaving an open gap around the furnace betweenthe top of the furnace walls and the cover, the steps of operating thefurnace with a pressure above atmospheric pressure under the cover atthe level of the gap, and confining the hot gases from escaping throughthe gap by maintaining a thin curtain of moving air across said gaparound the entire perimeter of the gap in such manner as to form apressure barrier in the gap between the air curtain and the innerperiphery of the gap with the air flowing to the outer periphery of thegap.

10. The method defined in claim 9 wherein there is a decreasing pressuregradient from the cover downwardly and the pressure under the cover ismaintained sufficiently high that the pressure inside the furnace isabove atmospheric pressure to a level close to the bottom of thefurnace.

11. The method defined in claim 9 wherein there is a decreasing pressuregradient from the cover downwardly and the pressure under the cover ismaintained sufficiently high that the level where the pressure insidethe furnace is at least atmospheric pressure at the lowest point in thefurnace.

12. The method of operating a soaking pit as defined in claim 9 in whichthe pressure in the furnace at the level of the gap is in a rangebetween about .05 and 0.15 inch of Water.

13. In a furnace having a bottom and vertical refractory walls enclosinga pit with the tops of the vertical walls providing a rim surface aroundthe entire periphery of the furnace, the furnace having a refractorycover for the pit supported for movement into and out of operativeposition over the pit, the cover having a peripheral surface whichextends over the rim surface of the side Walls in spaced confrontingrelation thereto when the cover is in operative position providing aperipheral gap between said surface, the invention comprising:

(a) a duct means extending around the furnace arranged to discharge aneffectively continuous curtain of air therefrom across the gap from oneconfronting surface toward the other at a velocity suflicient to impingeagainst the opposite confronting surface and at an inwardly-directedangle such that it impinges said confronting surface between the innerand outer boundaries of the gap, whereby the kinetic energy of the airis converted to pressure to block the escape of furnace gases while theair escapes outwardly from the gap,

(in) and means for supplying a controlled flow of air to said ductmeans.

14. The invention defined in claim 13 wherein said duct means isrecessed from direct exposure to heat radiated through the gap from theinterior of the furnace.

15. A furnace as defined in claim 13 wherein said duct is located on thecover around the exterior thereof and has air discharge openings thereinarranged to project a curtain of air across the gap at the outerperiphery of the gap but angled inwardly.

References Cited UNITED STATES PATENTS Cummings 263-50 Kephart 263-50Ritter 263-50 Browning 263-50 X West 110-173 West 263-43 Vedder 263-50JOHN J. CAMBY, Acting Primary Examiner.

